1. Field of the Invention
Fiber optic interface junction assembly.
2. Description of the Prior Art
In working with optical systems that use single fiber waveguides, it is frequently necessary to terminate the fiber in optical-coupling proximity to an interface of another optical member or device in the system. Because of the small size and frailty of single fibers, the methods and means needed to accomplish such optical junction need to be special.
Typical examples of devices or members having an interface surface to which a single fiber waveguide may be coupled optically include, for example, solid state detector or emitter windows, total or partially reflective mirrors, etc.
In one case, for example, a relatively high level of optical power is coupled in and out of one end of a coil of fiber optic waveguide that is terminated at its opposite end by a mirror interface surface to act as a passive optical range simulator device, as disclosed in copending U.S. Patent Application Ser. No. 775,061 filed Mar. 7, 1977 and assigned to the assignee of the present application. Coupling efficiency between the end of the waveguide fiber and the mirror interface surface is adversely affected by any separation distance. Although this source of loss can be virtually eliminated by resort to direct contact of fiber end with the mirror interface, techniques employed in the prior art are not readily adapted to such direct contact, primarily because of a common practice of embedding the fiber ends in rigid termination structures. A rigid termination structure being one in which the fiber end and its mechanical terminator are rigidly attached to one another by means of an epoxy casting that leads to optical polishing of the epoxy-encircled end surface of the optical fiber. Such technique is attended by potential problems which manifest to a greater or lesser degree according to specific circumstances of use. First, it is difficult to ensure that the embedded fiber axis coincides with the axis of the terminator assembly. If the fiber axis is tilted with respect to the terminator axis, polishing produces an end surface of the fiber that is tilted with respect to perpendicularity relative to the fiber axis, and this increases the probability of optical loss. Strain produced during the setting of casting resins can induce permanent microbending loss in the affected section of the fiber, and the absence of strain relief where the fiber enters/leaves the rigid embedment increases the chance of fracture at that stress point. Although the rigid embedment facilitates optical polishing operations, the result is that the fiber end aperture and the embedment surface lie in a common plane which is normally the plane of primary focus of optical energy being coupled into the fiber. If this energy is at a high enough level, there is a strong possibility that spillover energy at the fiber's periphery will burn the embedment material and contaminate the fiber end surface. Further, when a rigidly mounted fiber is brought into contact with a rigidly mounted interface surface, even very small assembly forces result in very high contact pressures (pounds per square inch) due to the small contact area. Damage to the interface surface (mirror, for example) and/or fiber end surface readily occurs, and in situations where the fiber is required to extend unsupported by a small distance beyond the surface of its terminator, there is a tendency for slight misalignments to induce flexure in such unsupported region which, when coupled with compressive forces, results in immediate fracture of the fiber extension. A common practice aimed at overcoming some of these difficulties has been to use shims or other mechanical means to provide controlled separation of the surfaces. Small separations are not easily achieved by such shimming technique, and the separation distance is affected by assembly pressures, material compliances, differential expansion effects, etc., with the result that coupling loss can become significant and unpredictable.